In general, fine pattern formation is carried out by photolithography in the manufacture of a semiconductor device or the like. In a fine pattern transfer process where the photolithography is carried out, a photomask is used as a mask. The photomask is generally obtained by forming a desired fine pattern in a light-shielding film or the like of a photomask blank as an intermediate product. As a consequence, the properties of the light-shielding film or the like of the photomask blank as the intermediate product almost exactly determine the performance of the obtained photomask. Conventionally, Cr has generally been used as the light-shielding film of the photomask blank.
In the meantime, in recent years, pattern miniaturization has advanced more and more and, following this, a problem such as resist collapse has arisen with the thickness of a conventional resist film. This will be explained hereinbelow. In the case of a light-shielding film mainly composed of Cr, either wet etching or dry etching can be used as etching after forming a transfer pattern in a resist film by EB (electron beam) writing or the like. However, in the case of the wet etching, the etching tends to proceed isotropically and thus it has become difficult to cope with the recent pattern miniaturization while the dry etching that tends to be anisotropic has become dominant.
In dry-etching a light-shielding film mainly composed of Cr, a mixed gas of chlorine-based gas and oxygen gas is generally used as an etching gas. However, a conventional resist film of an organic material has a property of being easily etched with the oxygen gas and thus the etching rate of the organic material resist film is much higher than that of the light-shielding film mainly composed of Cr. Since the resist film should remain until completion of patterning, by dry etching, of the light-shielding film mainly composed of Cr, the thickness of the resist film in the case of the light-shielding film mainly composed of Cr is required to be very large (e.g. three times the thickness of the light-shielding film mainly composed of Cr).
In recent years, the pattern miniaturization has been significant and there have been instances where, in a resist film having a transfer pattern formed by EB writing or the like, the height or thickness of the resist film is much greater than the width thereof at its portion where the pattern is dense, so that the resist film collapses or is stripped due to its instability during development or the like. If this occurs, the transfer pattern is not correctly formed in a light-shielding film mainly composed of Cr, thus resulting in a defective photomask. Therefore, the reduction in thickness of the resist film has been the overriding subject to be solved. For reducing the thickness of the resist film in the case of the light-shielding film mainly composed of Cr, it has been necessary to reduce the thickness of the light-shielding film. However, for the light-shielding film mainly composed of Cr, its thickness has already reached a limit where the light-shielding performance becomes insufficient.
JP-A-2006-78825 (Patent Document 1) discloses that a Ta metal film has an extinction coefficient (light absorbance) equal to or greater than that of a Cr metal film for light having a wavelength of 193 nm which is used in ArF excimer laser exposure. Patent Document 1 further discloses a photomask blank that enables precise formation of a fine photomask pattern by reducing the load to a resist which is used as a mask in the photomask pattern formation. This photomask blank comprises a light-shielding layer in the form of a metal film that is not substantially etched by oxygen-containing chlorine-based ((Cl+O)-based) dry etching, but can be etched by oxygen-free chlorine-based (Cl-based) dry etching and fluorine-based (F-based) dry etching, and an antireflection layer in the form of a metal compound film that is not substantially etched by oxygen-free chlorine-based (Cl-based) dry etching, but can be etched by at least one of oxygen-containing chlorine-based ((C+O)-based) dry etching and fluorine-based (F-based) dry etching.
Normally, before forming a resist, a photomask blank is cleaned using cleaning water or a cleaning liquid containing a surfactant for the purpose of removing particles present on a surface of the photomask blank. Further, in order to prevent the occurrence of stripping or collapse of a fine pattern in later processes, surface treatment is carried out for reducing the surface energy of the photomask blank surface. As the surface treatment, the photomask blank surface is, for example, alkyl-silylated with hexamethyldisilazane (HMDS) or another organic silicon-based surface treatment agent.
A defect inspection of the photomask blank is carried out before or after forming the resist and, if the desired specification (quality) is satisfied, a photomask is manufactured through later-described processes. The resist formed on the photomask blank is subjected to writing, development, and rinsing and, after forming a resist pattern, an antireflection layer is patterned by oxygen-containing chlorine-based dry etching or fluorine-based dry etching using the resist pattern as a mask to thereby form an antireflection layer pattern. Then, using the antireflection layer pattern as a mask, a light-shielding layer is patterned by oxygen-free chlorine-based dry etching to thereby form a light-shielding layer pattern, and finally the resist is removed. In this manner, the photomask is manufactured. The manufactured photomask is subjected to an inspection using a mask defect inspection apparatus to check whether or not there is a black or white defect and, if the defect is detected, correction is properly performed.